Z80 SBC with toggle switches

[Mark T]

I wanted to try and debug an old z80 board where I suspected the EPROMs were erased due to old age. I was originally planning to build a header board to plug into the z80 socket and use RC2014 bus to add single step, bus monitor and toggle switches to examine and modify memory. Looking through old magazine articles I found a review of the scrumpi sc/mp board in practical electronics August 1977 and decided to build something similar.

Single step circuit and z80 provide the address to memory.

Buffers drive LEDs to display address and memory content.

Toggle switches control RUN or single step, Write to memory or skip to next address.

Toggle switches to enter byte to write to the addressed memory.

It all fits into 100mm x 100mm, has a header to plug into a target z80 board and an RC2014 bus at the top edge. I was running out of space so I've mounted resistors on the reverse side of the board to save space on the top side for surface mounted LEDs.

Toggle switches are a tight push fit into the board so I haven't soldered these yet, its easier to remove the switches to make modifications to the board.

I've had this board working on its own, using a 6116LP for 2K RAM, which appears twice in the first 4K of the z80 address space. Options would allow 8Kx8 or 32Kx8 ram.

The single step circuit uses a 74LS123, dual monostable instead of a 74LS74 and 74LS121 to reduce the package count. I discovered in testing that the 74LS123 will retrigger at the end of the clear pulse, so increased the delay of the monostable to approx 400ms. This now work as an auto step if the skip or step switches are held closed.

There are a couple of issues that I'd probably try and fix if I make a second revision to avoid the wire mods.

1). The alter/skip switch briefly shorts the z80 side of IC6 74LS245 to ground before the direction control is changed.

2). When the write switch is activated both /RD and /WR are active. Most ram devices allow the /WR to override the /RD.